1. Field of Invention
This invention relates to a method for the fabrication of a high resistance load resistor in a highly integrated semiconductor memory device, and more particularly, to a method for fabricating a high resistance load resistor which has high resistance by utilizing polysilicon spacer on the sidewall of dielectrics layer. The resulting resistor can be used in a static memory device.
2. Information Disclosure Statement
In the case of the mega bit capacity SRAM in which a memory cell comprises four transistors and two load resistors, a load resistor having high resistance is required in order to reduce the stand-by current. However, in the case where a lightly doped polysilicon is utilized to make a high resistance load resistor, there is a physical limitation to the length of the resistor so that a high resistance load resistor cannot be obtained since the given cell size is too small in the mega bit static RAM device.
In the prior art, as the density of SRAM is increased, the size of chip is reduced. Therefore, the size of the unit cell should be reduced. Similarly, since there is such limitation to the size of the cell, there is a trend toward a static storage cell which utilizes two load resistors and four transistors rather than six transistors, for more highly integrated devices.
In order to achieve a low stand-by current, lightly doped polysilicon having a high resistance has been used as a load resistor. The resistance of the slightly doped polysilicon depends upon the thickness, the degree of doping and the length of the polysilicon. Accordingly, in the prior art method, the resistor having a few hundred Giga ohms can be obtained by optimizing these parameters, as mentioned above.
However, the length, which the polysilicon resistor can occupy, is extremely limited in the prior art as the density of the devices is increased to mega bits. Therefore, when the fact that the resistance is proportional to the length of material, is considered, it is very difficult to increase the resistance up to the degree of Tera ohm. Of course, an approach to increase the resistance of the polysilicon resistor by decreasing it's thickness to, for example, a few thousand angstroms utilizing multilayered polysilicon may be considered, but this also presents several problems in conjunction with a subsequent step in the manufacturing process.
Therefore, it is an object of the present invention to solve the drawbacks of the above mentioned prior art memory devices, and to provide a method for the fabrication of a high resistance load resistor having a very high resistance, in which more than Tera ohms can be achieved by a self-aligned polysilicon spacer by a anisotropic etching process in a small area of the cell. According to an aspect of the present invention, higher resistance can be provided in a limited length and the problems resulting from forming the multilayered polysilicon can be avoided.
It is a further object of the present invention to provide a high resistance load resistor having a high electrical resistance of, for example, Tera ohms, which can lower the stand-by current, and to provide a resistor which comprises a first highly doped polysilicon for conduction and a second lightly doped polysilicon for resistance thereon with both being connected by ohmic contact.
A further aspect of the present invention is to provide a high resistance load resistor which can be achieved without design rule limitation in manufacturing the highly integrated semiconductor device.